Mitochondria, the organelle found in almost all eukaryotic cells, play a vital role in the life and death of cells. Each mitochondria is composed of two concentric membranes, the inner membrane forming a series of folds that partially divide the interior matrix into communicating compartments. Mitochondria are generally ovoid or elongated in configuration; and uniquely contain their own genomic DNA, conventionally termed “mrDNA”, which is circular in its three dimensional structure and constitutes a type that is chemically separate and distinct from the chromosomal DNA in the cell's nucleus.
The most prominent function of mitochondria is to produce ATP, the energy currency of the cell. The production of ATP involves a series of electron transport systems in the oxidation phosphorylation pathway, which is also found to be associated with the generation of reactive oxygen species (ROS). The production of ROS in mitochondria leads to the propagation of free radicals, damaging cells, and contributing to cell death, which is known as mitochondria-mediated apoptosis.
The morphology of mitochondria, though variable based upon cell type, cell-cycle stage, and intracellular metabolic state, is affected by and thus reflects cell functioning. The morphology is controlled by a set of proteins, mutations of which will cause several human diseases including degenerative diseases such as Parkinson's and Alzheimer's diseases. Recent reports also show that proteins participating in apoptosis can affect the morphology of mitochondria. Tracking the mitochondrial morphological change may give insight for studying apoptosis and degenerative conditions. The need to visualize and monitor mitochondria inside the cell is known.
The importance of monitoring the morphology, functionality, activity, and number of the mitochondria is associated with the possibility of proving the harmful effects of various substances and of showing mutations in the mitochondrial DNA. In the first case, for example, some anaesthetics have proved to be modifiers of mitochondrial activity (Biochem. J. 1990, 271, 269), while in the second case it has been proved that mutations of the mitochondrial DNA are often implicated in a number of neurodegenerative diseases such as Parkinson's and Alzheimer's and involve cell death by apoptosis or necrosis.
Fluorescent probes used to detect important biological events in living cells or animals have been in increasing demand in the biological and biomedical fields over the past two decades. Many kinds of fluorescent bioprobes have been developed, such as organic dyes, inorganic nanoparticles, and fluorescent polymers.
In this regard, fluorescent probes that can selectively illuminate cellular mitochondria are powerful tools for monitoring the morphological changes and studying these processes. To successfully observe the dynamic changes in a certain period of time, the probe must be photostable under the continual irradiation of light from fluorescent microscopes. Conventional fluorescent dyes for mitochondria staining have been developed. Their photostability, however, leaves much to be desired. Very diluted solutions of these dyes are used in the imaging process and such small numbers of the dye molecules can be quickly photobleached when a harsh laser beam is used as the excitation light source. The photostability cannot be improved by using higher fluorophore concentration due to the accompanying concentration-quenching effect.
Mitochondria targeting probes with different functions have been reported in the prior art, examples of which have been reported by Abbotto Alessandro (WO 2007/113321 A1), Zarling David A. (WO 2008/109740 A2), Dario C. Altieri (US 2009/0099080 A1), and Shibnath Ghosal (US 2008/0031862 A1). Generally, these previous disclosures face several problems, for example losing the specificity to mitochondria once the mitochondrial membrane potential is collapsed, suffering from aggregation-caused quenching, or even only having a complicated synthetic route. Accordingly, there remains a need in the art to address these issues.